1. Field of the Invention
The present invention relates to a multilayer wiring board in which a plurality of metal conductor pattern layers are formed on a base material made of a thermoplastic resin and a method of fabricating the same.
2. Description of the Related Art
Along with the widespread use of portable electronic devices such as a mobile phone, a digital still camera, build-up boards have come to the forefront in the market. The build-up boards are designed to realize ultra-miniaturization of components as well as a fine pattern or miniaturization of via hole sizes for high-density implementation. However, the build-up board is produced based on so-called a sequential multilayer wiring technology in which respective layers are sequentially formed and laminated, complicating a board fabrication process.
Under the circumstance, a method that has been applied to a ceramic multilayer board using a green sheet is now being applied also to an organic material. In this method, via holes, wiring patterns, and the like are previously formed for each layer and are positioned between layers, followed by a batch lamination process.
As the organic base material, a thermoplastic resin is very useful. The reason is as follows. A wiring board made of a thermoplastic resin does not have to be subjected to vacuum hot press for a long time for removal of voids contained in the base material or resin hardening, which is necessary in the case of a glass-fiber reinforced thermoset resin such as a widely used glass epoxy, and bonding between the base material and conductor pattern by heat allows batch lamination process in a short period of time and following integration process. As a result, it is possible to obtain a multilayer structure in a simpler manner. Further, it is possible to select a base material excellent in high-frequency characteristics such as dielectric constant.
As a technique that can be applied to interlayer connection of a wiring in a batch-fabricated multilayer, there is available a technique for obtaining electrical connection by filling via holes with a metal paste. For example, Japanese Patent No. 3473601 discloses such a technique. In this technique disclosed in Japanese Patent No. 3473601, via holes formed in a thermoplastic resin material on which wiring patterns are formed are filled with tin which is a low melting metal and alloyed with a metal forming a conductor pattern and silver powder which is a high melting metal having a melting point higher than the heat-application temperature at the time of interlayer connection to establish electrical connection between the conductor patterns through an alloy layer of the metal forming the conductor pattern under heating and pressurization processes and tin contained in a conductive composition obtained by sintering. As described above, in this technique, electrical connection between conductor patterns is made by means of conductive compositions.
The electrical connection between conductor patterns is not made by electrical connection process in the above connection method, so that an interlayer connection resistance is unlikely to change, preventing the reliability of the interlayer connection from being lowered.
In the alloying process, as shown in a schematic view of FIG. 1, heat is applied at a temperature at which a low melting metal particle 101 packed in a via hole formed in a base material 100 is melt to induce alloying between the melted low melting metal and a high melting metal 102 in a sequential manner starting from the interface of the high melting metal 102. As a result of the alloying process between the low melting metal particle 101 and high melting metal particle 102, an alloy (intermetallics) 103 is formed between the metal particles. So-called sintering occurs. At this time, at the interface between a metal wiring pattern 104 and the alloy 103, an alloy (intermetallics) layer 105 of the wiring pattern and low melting metal is formed, thus connecting the metal wiring patterns 104 between layers.
The alloys 103 and 105 have a high melting point, so that even when a temperature as high as 250° C. or more, which is necessary for soldering, is applied, a conductive composition is not melted and the strength thereof can be maintained. For example, if silver (melting point: 961° C.) is used as a high melting metal and tin (melting point: 232° C.) is used as a low melting metal in the technique disclosed in Japanese Patent No. 3473601, an obtained alloy of the silver and tin has a melting point of 480° C., thus increasing the melting point. The formation of the alloys (intermetallics) 103 and 105 allows enough strength to resist a heat treatment process performed at the soldering time in the post-process to be sufficiently secured.